In recent years, hydroforming technology has been the focus of attention in the automobile field as one of the means for reducing the number of parts and thereby cutting costs, for lightening the weight, etc. In Japan as well, it began to be applied to actual cars starting in 1999. Since then, the parts which can be hydroformed have increased. The market has greatly expanded in size.
There are many advantages to hydroforming other than the above reduction of the number of parts and the lightening of the weight. For example, the fact that piercing of a metal pipe is possible at the same time as hydroforming may be mentioned. An outline of that technology is as shown in FIG. 1. In this technology, a pierce punch 6 assembled into the hydroforming mold (in the case of this example, the upper mold 2) is being pushed in the direction of the mold cavity, so a hole is formed in the hydroformed part 1. At that time, the high pressure internal pressure causes the hydroformed part 1 to be pushed against the mold 2, so the edges of the hole will almost never droop down to the inner surface side and a good cut surface can be obtained. Further, the pierced metal piece 7 is sometimes completely punched out as shown in FIG. 1(a), but sometimes, as shown in FIG. 1(b), part is not cut and the piece left. Specifically, by partially making larger the chamfering of the edges of the front end of the pierce punch, cutting of that location is prevented.
There are many advantages to hydroforming as mentioned above, but as a defect, the point that joining with other parts is difficult may be mentioned. In the case of a conventional press formed part, it has been fastened to another part 11 by spot welding such as shown in FIG. 2(a) or bolting such as shown in (b). However, a hydroformed part is hollow, so spot welding was difficult. Further, attachment of a nut inside was also impossible. As shown in FIG. 3, the hydroformed part 1 may have a nut 13 welded to its outer surface side, but the nut 13 sticks out from the outer surface side of the hydroformed part 1, so when joined with another part 11, the parts cannot contact each other at their surfaces.
As an example of attachment of a nut to a hydroformed part, there is Japanese Patent Publication (A) No. 2002-45926. As shown in FIG. 4, this method performs the hydroforming by wrapping around the outer surface of the hydroformed part 1 a short pipe member 61 having a bag nut 60 attached to it.
Further, as another example of attaching a nut to the hydroformed part, there is Japanese Patent Publication (A) No. 2003-334625 as shown in FIG. 5. The main difference between the present method and the above-mentioned Japanese Patent Publication (A) No. 2002-45926 is the point that the nut 13 is sandwiched between the short pipe member 61 and the hydroformed part 1.
Further, as other prior art, there is Japanese Patent Publication (A) No. 2005-297060. This method, as shown in FIG. 6, is a method which forms a burled part 62 at the inner surface side of the hydroformed part, takes the part out from the hydroforming mold, then cuts a tap 63 at the burled part and uses a bolt 64 to join the part with another part 65.